Cytoplasmic inheritance of temperature sensitivity in a wild stock of Paramecium primaurelia

One stock, GBC, has a maximum temperature of growth about 5 degrees C lower than other recently collected stocks of Paramecium primaurelia. The resistant stocks (R) are able to grow continuously at 35 degrees C while the sensitive stock (S) cells die within 48 h. The F1s of R X S crosses exhibited a cytoplasmic pattern of inheritance and all F2-by-autogamy lines derived from the S cytoplasmic parent are sensitive. The F2-by-autogamy lines cytoplasmically descended from the R parent were predominantly (93%) R in the initial assay. Upon reinvestigation one year later, only 64% of these lines were R, 9% were S, and 27% had a new phenotype, weak (W), intermediate between R and S. Backcrosses of W lines to both R and S strongly suggest that the W lines have normal cytoplasm (i.e. R) but also have nuclear gene(s) for temperature sensitivity that are derived from the original S stock. The delayed manifestation of the W phenotype is not understood.